Water is the most important natural resource in the world. The water may be fresh or brackish, existing as surface water or ground water. Water resources form an integral part of the wetland ecosystems. Man depends on surface water sources and ground water aquifers for drinking, domestic, agricultural, industrial, navigational and other recreational purposes.
Clean and good quality water is highly essential for all living beings. The presence of safe and reliable source of water is thus an essential prerequisite for the establishment of a stable community. Now days, clean water has become a precious commodity and its quality is threatened by numerous sources of pollutants or pollution. Rapid industrialization, urbanization, and other manmade activities are the main causes of water pollution. Water is essential for the man land which keeps life going. There are many sources from which we fetch water. One such source is ground water.
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Ground water plays major role in places where there is no dependable source. The presence of fluoride more than 1.5 mg/lit will cause health hazard. Fluoride is found in all natural waters at some concentration.
In groundwater's, however, low or high concentrations of fluoride can occur, depending on the nature of the rocks and the occurrence of fluoride-bearing minerals. Concentrations in water are limited by fluorite solubility, so that in the presence of 40 mg calcium it should be limited to 3.1 mg (Hem, 1989). It is the absence of calcium in solution which allows higher concentrations to be stable (Edmunds and Smedley, 1996). High fluoride concentrations may be expected in groundwater from calcium-poor aquifers and in areas where fluoride-bearing minerals are common. Fluoride concentrations may also increase in groundwater in which cation exchange of sodium for calcium occurs (Edmunds and Smedley, 1996).
Fluorine, a fairly common element of the earth's crust, is present in the form of fluorides in a number of minerals and in many rocks. Fluorine in the environment is therefore found as fluorides which together represent about 0.06-0.09 per cent of the earth's crust. The average crustal abundance is 300 mg kg/1 (Tebutt, 1983). Fluorides are found at significant levels in a wide variety of minerals, including fluorspar, rock phosphate, cryolite, apatite, mica, hornblende and others (Murray, 1986). Fluorite (CaF2) is a common fluoride mineral of low solubility occurring in both igneous and sedimentary rocks. Fluoride is commonly associated with volcanic activity and fumarolic gases. Thermal waters, especially those of high pH, are also rich in fluoride (Edmunds and Smedley, 1996).
Fluoride enters into water due to both natural processes and human activity. Since fluoride is present in several minerals, it can be leached out by rainwater thereby allowing it to contaminate ground and surface water. On the other hand several fluoride compounds have industrial applications and these also contribute to fluoride pollution. Fluoride is frequently encountered in minerals and in geochemical deposits. Because of the erosion and weathering of fluoride-bearing minerals it becomes a surface species. On the other hand, fluorine compounds are industrially important and are extensively used in semiconductors, fertilizers, aluminium industries, and nuclear applications.
Toxic wastes containing fluoride are generated in all industries using fluorine or its compounds as raw materials. Prominent among these is the aluminium smelter where fluoride gas is released into the atmosphere. Fluoride is an essential constituent for both humans and animals. Within permissible limits of 0.5-1.0 mg/L, is beneficial for the production and maintenance of healthy bones and teeth, while excessive intake of fluoride causes dental or skeletal fluorosis which is a chronic disease manifested by mottling of teeth in mild cases, softening of bones and neurological damage in severe cases.
Fluoride content can vary greatly in wells in the same area, depending on the geological structure of the aquifer and the depth at which water is drawn. Deepening tubewells or sinking new wells in another site may solve the problem. The fact that fluoride is unevenly distributed in groundwater, both vertically and horizontally means that every well has to be tested individually for fluoride in areas endemic for fluorosis (Schoeman and MacLeod, 1987). The fluoride-bearing minerals or fluoride-rich minerals in the rocks and soils are the cause of high fluoride content in the groundwater, which is the main source of drinking-water.
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Exposure to high levels of fluoride can cause endemic fluorosis, dental fluorosis, skeletal fluorosis, non-skeletal fluorosis, arthritis, cancer etc. Stiff joints, weight loss, brittle bones, anemia and weakness characterize endemic fluorosis. Discolored, blackened, white teeth characterize dental fluorosis, skeletal fluorosis leads to gastro - intestinal problems and neurological disorders. Fluoride can damage a fetus and adversely affect the IQ of children.
Defluoridation is the process of removal of fluoride ion in drinking water. The process may be classified broadly into two categories, namely, I) Additive methods, and II) Adsorptive methods. Some defluoridation techniques developed to control fluoride content in water are reverse osmosis, adsorption using sunflower plant dry powder, steam of phytomass, Holly Oke, neem bark powder, activated cotton jute carbon, bagasse ash, burnt bone powder, phosphate-treated saw dust, bone char, etc. as adsorbents, Nalgonda technique, activated alumina process and ion exchange process. However, due to high cost or lower efficiency or non-applicability on mass scale these techniques are not much in use.
Few natural materials such as red soil, untreated charcoal, local powdered brick, fly-ash (thermal power plant) and mineral serpentine were used. The ability of soil to absorb fluorine from solution has been studied by earlier researchers (Bower and Hatcher, 1967; Fluker et al., 1982; Gupta et al., 1982). Various methods and techniques have been applied and used for the treatment of access fluoride in drinking water. Each of them has certain advantages and disadvantages and their individual appropriateness would always depend on the actual local conditions, the most important factor being the community's acceptance.
However, most of these methods have high operational and maintenance cost, low fluoride removal capacities, lack of selectivity for fluoride, undesirable effects on water quality, generation of large volumes of sludge and complicated procedures involved in the treatment. Adsorption is the process considered to be efficient to defluoridate the water. Researches were carried on different adsorbents, viz. activated carbon, processed bone char powder, activated alumina, magnesia, activated bauxite, fly ash, granular calcite, alum, lime, etc. The adsorption process is a widely accepted pollution-removal technique, because of its ease of operation and cost-effectiveness. Different types of adsorbents such as natural, synthetic, and biomass, are used for removal of fluoride from water.
The different methods so far tried for the removal of excess fluoride from water can be broadly classified into four categories: A) Adsorption methods, B) Ion exchange methods, C) Precipitation methods, and D) Miscellaneous methods:
Fluoride can be removed by adsorption onto many adsorbent materials. The criteria for selection of suitable sorbents are: cost of the medium and running costs, ease of operation, adsorption capacity, potential for reuse, number of useful cycles and the possibility of regeneration. It is carried using different adsorbents i.e. activated alumina, clays and soils
Activated alumina is a granular form of aluminium oxide (Al2O3) with very high internal surface area, typically in the range of 200-300 m2/g. This high surface area allows the material a very large number of sites where adsorption can occur. It has been widely used for removal of F from drinking water (Hao et al., 1986; Schoeman and MacLeod, 1987). The mechanism of F removal from water is similar to those of a weak base ion exchange resin. Fluoride removal efficiency is excellent (typically > 95%), and is dependent on pH. Fluoride removal capacity is best in the narrow range of pH 5.5 to 6. Fine (28-48 mesh) particles of activated alumina are typically used for F removal.
Activated alumina can be regenerated by flushing with a solution of 4% sodium hydroxide which displaces F from the alumina surface (Schoeman and MacLeod, 1987). This procedure is followed by flushing with acid to re-establish a positive charge on the surface of the alumina.
Clays and soils.
The first comprehensive study of fluoride adsorption onto minerals and soils was published in 1967 (Bower and Hatcher, 1967). Since the above -mentioned paper was published; several workers studied the adsorption of fluoride. These studies include the use of Ando soils of Kenya ( Zevenbergen et al., 1996), Illinois soils of USA (Omueti and Jones, 1977), Alberta soil (Luther et al., 2996), illite- goethite soils in China (Wang and Reardon, 2001), clay pottery (Chaturvedi et al., 1988; Hauge et al., 1994), fired clay (Bardsen and Bjorvatn, 1995), fired clay chips in Ethiopia (Moges et al., 1996), kaolinite (1997), bentonite and kaolinite (Kau et al., 1998; Srimurali et al., 1998), and fly ash (Chaturvedi et al., 1990).
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Several researchers have studied the removal of F using fired clays (Hauge et al., 1994; Bardsen and Bjorvatn, 1995; Moges et al., 1996). Hauge et al (1994) studied the defluoridation of drinking water using pottery. The study investigated the effect of temperature on F adsorption. The results show that clays fired at temperature up to 600°C gave higher F adsorption. Moges et al (1996) studied the defluoridation of water using fired clay chips in Ethiopia. Their findings indicated that F adsorption is affected by factors such as initial concentration, mass of adsorbent and the pH of the solution.
Low cost materials.
Srimurali et al. (1998) investigated the removal of F using low cost materials such as kaolinite, bentonite, charfines, and lignite. Their results show that F adsorption using nirmali seeds and lignite is low (6 to 8%). The removal of F by kaolinite is slightly better (18.2%) while charfines and bentonite give higher F removal capacity of 38 and 46% respectively. Chemical pretreatment was used to investigate its effect on the removal capacity of these materials. Kau et al. (1998) investigated the adsorption of F by kaolinite and bentonite. The results show that bentonite was found to have higher F adsorption than kaolinite.
Fluoride removal using China clay was studied (Chaturvedi et al., 1988). The results show that low F concentration, high temperature and acidic pH are factors favouring the adsorption of F. It was concluded that the alumina constituent of the China clay is responsible for F adsorption. Chaturvedi et al. (1990) also studied the defluoridation of water by adsorption on fly ash. This study confirms their previous findings that low F concentration, high temperature and acidic pH favour the adsorption of F.
Several studies investigated the adsorption of F using soils (Omueti and Jones, 1977; Chhabra et al., 1980; Zevenbergen et al., 1996; Bjorvatn et al., 1997; Wang and Reardon, 2001). Chhabra et al. (1980) investigated the effect of varying levels of exchangeable sodium on the adsorption of F onto sodic soils. It was noted that at equilibrium F concentration, a decrease in adsorption of F with increase in the soil exchangeable sodium percentage was observed. Omueti and Jones (1977) studied the adsorption of F by Illinois soils. They reported that at low concentrations F adsorption onto soils was described by both Langmuir and Freundlich isotherms. It was also suggested that F adsorption onto soils was due to the presence of the amorphous aluminium hydroxides. Bjorvatn et al. (1997) studied the defluoridation of water using soil samples from Ethiopia. It was reported that five soil samples from highland areas around Addis Ababa reduced the fluoride content of the water from about 15 to 1 mg.L-1. From this study, it was concluded that the highland soil may be useful for removal of excessive F from drinking water.
In addition to activated alumina, clays and soils other materials such as spent bleaching earth, spent catalyst, rare earth oxides, bone charcoal and activated carbon were studied as sorbents for F. Mahramanlioglu et al (2002) investigated the adsorption of F using spent bleaching earth. They found that the removal of F depends on the contact time, pH and adsorbent concentration. Lai and Liu (1996) studied the F removal from water with spent catalyst. Their findings showed that spent catalyst could be utilized as adsorbent for F removal. Its adsorption capacity was comparable to that of activated alumina. Raichur and Basu (2001) studied the adsorption of F onto rare earth oxides. Rare earth oxides showed great potential for F removal from water. Lu et al (2002) investigated the removal of F using red mud. The removal of F using red mud was found to be 82%.
Advantages of adsorption methods.
• The process can remove fluoride up to 90%.
• Treatment is cost-effective.
Limitations of adsorption methods.
• The process is highly dependent on pH and works best only in a narrow pH range (5-6).
• High concentration of total dissolved salts (TDS) can result in fouling of the alumina bed.
• Presence of sulfate, phosphate or carbonate results in ionic competition.
• The process has low adsorption capacity, poor integrity and needs pretreatment.
• The regeneration is required after every 4-5 months and effectiveness of adsorbent for fluoride removal reduces after regeneration.
• Disposal of fluoride laden sludge and concentrated regenerate is also a problem.
Commonly used techniques, adsorption was shown to be most efficient. However, the adsorption capacities of most cost-efficient adsorbent materials, such as zeolites, bentonite, alum sludge, and red mud, decrease sharply under saline conditions. In addition, the sorption process is highly pH dependent. The most effective pH range is at pH < 3.0, and little removal of F− occurs at neutral pH (Srimurali et al., 1998)
B. ION EXCHANGE RESINS.
Ion exchange resins are effective in removing F from water. Mohan Rao and Bhaskaran (1988) studied the removal of F using ion exchange materials such as sulphonated material from coconut shell, Carbion, Tulsion and Zeocarb 225. From the results, it was evident that Zeocarb 225 has the highest F removal capacity and sulphonated material of coconut shell has the lowest. It was also indicated that the ion exchange material can be regenerated by aluminium sulphate solution (2-4%). Castel et al (2000) studied the removal of F by a two way ion exchange cyclic process. This system used two anion exchange columns. The results show that this process can effectively remove fluoride from water.
The use of anion exchange resins for F removal is not common because of their relatively high costs. The presence of other anions such as chloride and sulphate also presents a major problem when using ion exchange resins for F removal. Since F removal is accompanied by sorption of other anions, the sorption capacity is normally lower than 0.5 mg F.L-1 (Veressinina et al., 2001).
Advantages of ion exchange.
• Removes fluoride up to 90-95%.
• Retains the taste and colour of water intact.
Limitations of ion exchange.
• Efficiency is reduced in presence of other ions like sulfate, carbonate, phosphate and alkalinity.
• Regeneration of resin is a problem because it leads to fluoride rich waste, which has to be treated separately before final disposal.
• The technique is expensive because of the cost of resin, pretreatment required to maintain the pH, regeneration and waste disposal.
• Treated water has a very low pH and high levels of chloride.
Membrane processes such as reverse osmosis, nanofiltration and electrodialysis are recently developed methods for F removal from water (Schoeman and Steyn, 2000; Lhassani et al., 2001; Garmes et al., 2002). Not much research has gone underway using these membrane processes. However, the study by Lhassani et al (2001) indicates that F can be removed using nanofiltration.
Advantages of membrane processes.
• The process is highly effective for fluoride removal. Membranes also provide an effective barrier to suspended solids, all inorganic pollutants, organic micro pollutants, pesticides and microorganisms, etc.
• The process permits the treatment and disinfection of water in one step.
• It ensures constant water quality.
• No chemicals are required and very little maintenance is needed.
• Life of membrane is sufficiently long, so problem of regeneration or replacement is encountered less frequently.
• It works under wide pH range.
• No interference by other ions is observed
Limitations of membrane processes.
• It removes all the ions present in water, though some minerals are essential for proper growth, remineralization is required after treatment.
• The process is expensive in comparison to other options.
• The water becomes acidic and needs pH correction.
• Lot of water gets wasted as brine.
• Disposal of brine is a problem.
• The performance of all the above processes has been tested in the laboratory.
D. PRECIPITATION METHODS.
Precipitation methods can be divided into two categories, those based on co-precipitation of adsorbed F and those based on the precipitation of insoluble fluoride compounds.
Methods based on co-precipitation: Co-precipitation (e.g. the Nalgonda Technique) is the process by which aluminium salts (aluminium chloride and aluminium sulphate) is added to F contaminated drinking waters for treatment (Yang et al., 1999; Yang and Dluhy, 2002). This process is used in three ways.
· A bucket system designed to be used on household scale
· Fill and draw plants to be used on community scale
· A waterworks flow system developed for larger communities.
(I) Bucket system.
The bucket defluoridation system was first practiced for domestic use in Tanzania (Mjengera and Mkongo, 2002). The two chemicals (aluminium chloride and aluminium sulphate) are added simultaneously to the raw water bucket and stirred with a wooden paddle. Lime is added to adjust the pH of water to about 6.7. After addition of the chemicals it is left to settle for about 1 hour. This process is suitable for a daily routine, where one bucket of water is treated for one day's water supply of about 20 L. The process produces water with residual F between 1 and 1.5 mg.L-1 (Dahi et al.; 1996).
(ii) Fill and draw system.
This system is also used in Tanzania for the defluoridation of drinking water (Mjengera and Mkongo, 2002). It consists of a cylindrical vessel equipped with a hand operated stirring mechanism. The vessel is filled with raw water and a similar procedure for defluoridation using bucket system is performed. Raw water is pumped onto the tank and the required amounts of alum, lime and bleaching powder are added. The contents are stirred slowly for ten minutes and allowed to settle for two hours. The defluoridate supernatant water is withdrawn and supplied through stand-posts. The settled sludge is discarded.
(iii) Waterworks flow system.
A bigger defluoridation system is used for larger communities. This system involves the combined use of alum and lime for the defluoridation process (Dahi, 1996). It consists of several components, namely, reactors, sump well, sludge drying beds, elevated service reservoir, electric room and chemical storehouse. The raw water from the source is pumped to the reaction-cum-sedimentation tank which is referred to as reactor. A sludge pipe with sluice valve is provided to withdraw the settled sludge once a day.
The Nalgonda technique has been introduced in many countries, e.g. India, Kenya, Senegal and Tanzania. However, the method has a number of disadvantages. These include:
· The treatment efficiency is about 70%, which means the process cannot be used in cases of high fluoride contamination.
· A large dosage of aluminium sulphate, up to 700-1200 mg.L-1 may be needed.
· The adverse health effects of dissolved aluminium species in the treated water.
Elevated concentration of fluorides in water has detrimental effects to human health. Exposure to high levels of fluoride can cause endemic fluorosis, dental fluorosis, skeletal fluorosis, non-skeletal fluorosis, arthritis, cancer etc. Stiff joints, weight loss, brittle bones, anemia and weakness characterize endemic fluorosis.
Fluoride removal using activated alumina and reverse osmosis was proposed as a solution to the problem in South Africa (Schoeman and Steyn, 2000). However, the capital cost for household defluoridation was estimated to be about R 5000 for a 50 L per day unit, using either of the methods. This turned out to be too expensive. There is thus a need to develop low cost methods to remove fluoride from water. The removal of fluoride using locally available clays was studied in many countries where the problem occurs.
Therefore, it is important to make efforts to understand various possible approaches to improve the fluoride uptake properties of potential materials including clays. The present investigation deals with modification of bentonite clay with an electro positive atom, in order to enhance its adsorption capacity for fluoride ions from drinking water. It was well reported that the fluoride anions has stronger affinity for electro positive atoms. The clay materials like bentonite and kaolinite have been cited as low cost materials for defluoridation of water. However their major limitation is their low adsorption capacity. Hence, it will be useful to improve the adsorption capacity of bentonite clay for its possible use in defluoridation of drinking water.
The main objective of this research is to evaluate the efficiency of locally available South African industrial and natural clay to adsorb fluoride from an aqueous solution as alternative to existing commercial adsorbents. It is necessary to develop an understanding of the effect of the following variables in the removal of fluoride.
The effect of initial fluoride concentration of wastewater influent, and pH of influent
The effect of the amount of Fe modified bentonite on fluoride removal.
The effect of temperature on the removal of fluoride
To find an effective technique to treat wastewaters and groundwater.
To possibly develop a new market/areas of application for Fe bentonite in South Africa and neighboring countries.
Hypothesis is that sorbed fluoride can be released by increasing the temperature of the sorbent material.